Growing algae for biodiesel on direct sunlight or sugars: A comparative life cycle assessment

Growing heterotrophic algae in reactors with sugar as the energy and carbon source rather than sunlight and carbon dioxide is an approach being commercialized today. However, the full environmental impacts of this fuel pathway have not been explored. The objective of this analysis was to compare the life cycle impacts of algal biodiesel produced heterotrophically to a phototrophic pathway featuring algae grown in ponds. A third, hybrid approach utilizing algae capable of both phototrophy and heterotrophy was also explored. Sugar beet and sugarcane were examined as feedstocks for the heterotrophic process. The results indicate that a reduction in the global warming potential (GWP) and an improvement in the net energy ratio (NER) for algal biodiesel could be possible for the heterotrophic and hybrid pathways relative to the phototrophic, but only if reactor cultivation can be performed efficiently and with sugarcane as the feedstock. For example, the NER varies from 0.6 to 1.6 for the heterotrophic pathway, depending on reactor performance, compared to 1.3 for the phototrophic pathway. Sugar crops used as feedstocks for heterotrophic cultivation present concerns about land constraints that are less of an issue for the phototrophic pathway. No pathway presented a clear advantage for the water stress impact metric.